Method and apparatus for separating air by cryogenic distillation

ABSTRACT

In a method for separating air by cryogenic distillation, cooled air purified to remove water is sent to a first column operating at a first pressure, where it is separated into a nitrogen-enriched gas as an oxygen-enriched liquid; a gas enriched in argon relative to the air is withdrawn from the second column; at least a portion of the oxygen-enriched liquid is vaporized by heat exchange with the argon-enriched gas; and the vaporized, oxygen-enriched liquid is sent to an intermediate level of the second column.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of priority under 35 U.S.C. § 119 (a) and (b) to French patent application No. FR1911900, filed Oct. 24, 2019, the entire contents of which are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a method and an apparatus for improving the energy performance of a cryogenic air separation unit with or without an argon separation column.

BACKGROUND OF THE INVENTION

Double air distillation columns are well known in the art, and it is also common to combine them with an argon separation column.

Conventionally, in an air separation apparatus, air which has been purified and cooled is sent to a first column, operating at a cryogenic temperature, to be separated into a nitrogen-enriched gas and an oxygen-enriched liquid.

The liquid is withdrawn from the first column and sent to a second column, operating at a pressure lower than the first column, after expansion in a valve.

Air separation apparatuses often comprise an argon separation column in addition to the double column. This argon separation column may obviously serve for producing argon, although in certain cases the main purpose of its installation is to enhance the yield of oxygen and/or to enhance the production of nitrogen at a high pressure and/or to enable expansion of lots of air intended for the second column in order to increase the production of frigories and therefore the production of liquid, or to improve the energy performance.

A method according to the prior art is known from EP-A-0860670. In this method, the liquid feed to the overhead condenser of the argon column does not come directly from the first column, but has undergone partial vaporization beforehand in order to condense the argon mixture. As the liquid becomes more concentrated in oxygen, there is accordingly an increase in its vaporization temperature. The temperature difference in the argon column condenser is then excessively low and requires a very large exchanger volume. The consequence of this is to increase the size of the cold box.

SUMMARY OF THE INVENTION

One aim of the present invention is to improve the energy performance of air separation units with or without the argon separation column being present.

Where the argon separation column is present, even if argon is not produced and/or the column contains only very few stages, certain embodiments of the invention aim to reduce the additional cost linked to the presence of this column. Accordingly, the gain in energy performance provided by the invention can be garnered totally or partially at smaller extents.

According to one subject of the invention, a method is provided for separating air by cryogenic distillation, in which

a) cooled air purified to remove water is sent to a first column operating at a first pressure, where it is separated into a nitrogen-enriched gas and an oxygen-enriched liquid,

b) a liquid enriched in nitrogen relative to the air is withdrawn from the first column and sent to the top of a second column which is connected thermally to the first column and operates at a second pressure, which is lower than the first pressure,

c) a liquid enriched in oxygen relative to the air is withdrawn from the first column, and optionally a first portion of the oxygen-enriched liquid is sent to an intermediate level of the second column, optionally after having undergone a partial vaporization step in which it has been enriched in oxygen,

d) a gas enriched in argon relative to the air is withdrawn from the second column,

e) at least a portion of the oxygen-enriched liquid is at least partly vaporized by heat exchange with the argon-enriched gas and the vaporized, oxygen-enriched liquid is sent to an intermediate level of the second column, optionally following a step of enrichment of the vaporized liquid with oxygen,

f) at least one condensed portion of the argon-enriched gas is returned to a third column, which is also fed with an argon-enriched gas flow originating from the second column, an argon-enriched flow is withdrawn at the top of the third column, and an argon-depleted liquid is returned from the third column to the second column.

g) a portion of the oxygen-enriched liquid is sent to an overhead condenser of the third column,

h) the oxygen-enriched liquid sent to the overhead condenser undergoes vaporization there, and the vapour produced is sent to the second column

wherein the portion of the oxygen-enriched liquid that is sent to the overhead condenser of the third column has not been reheated against the argon-enriched gas flow.

According to other aspects, which are optional and can be combined with one another:

-   -   the vaporized, oxygen-enriched liquid is at a pressure at least         1 bar greater than the pressure of the second column, and is         expanded in a turbine and then sent to an intermediate level of         the second column;     -   at least one condensed portion of the argon-enriched gas is         returned to the second column;     -   at least one condensed portion of the argon-enriched gas is         returned to a third column, which is also fed with an         argon-enriched gas flow originating from the second column, an         argon-enriched flow is withdrawn at the top of the third column         and an argon-depleted liquid is returned from the third column         to the second column;     -   the portion of the oxygen-enriched liquid that is sent to the         overhead condenser of the third column has not undergone         enrichment with oxygen;     -   the third column is disposed inside the second column and the at         least one portion of the oxygen-enriched liquid is vaporized by         heat exchange with the argon-enriched gas inside the second         column;     -   the argon-enriched gas sent to the exchanger has a condensation         temperature greater than the vaporization temperature of the         oxygen-enriched liquid in the exchanger;     -   all of the oxygen-enriched liquid is sent from the bottom of the         first column to the heat exchanger; only a portion of the liquid         is vaporized, and this portion is sent to the second column;     -   in this case, the unvaporized portion is separated in a phase         separator, expanded and sent to the second column;     -   a portion of the oxygen-enriched liquid is sent from the bottom         of the first column to the heat exchanger, in which it is at         least partly vaporized, and a portion of the oxygen-enriched         liquid is sent from the bottom of the first column to the second         column, without passing via the heat exchanger.

All of the oxygen-enriched liquid sent to the heat exchanger undergoes vaporization there.

According to another subject of the invention, an apparatus is provided for separating air by cryogenic distillation, comprising a first column operating at a first pressure, a second column connected thermally to the first column and operating at a second pressure, which is lower than the first pressure, a heat exchanger, means for sending cooled air, purified to remove water, to the first column, operating at a first pressure, where it is separated into a nitrogen-enriched gas and an oxygen-enriched liquid, means for withdrawing a liquid enriched in nitrogen relative to the air from the first column, means for sending the nitrogen-enriched liquid to the top of the second column, means for withdrawing a liquid enriched in oxygen relative to the air from the first column, optionally means for sending a first portion of the oxygen-enriched liquid to an intermediate level of the second column, optionally after enrichment thereof with oxygen, means for withdrawing a gas enriched in argon relative to the air from the second column, means for sending a portion of the oxygen-enriched liquid to the heat exchanger for at least partial vaporization thereof by heat exchange with the argon-enriched gas, and means for sending the oxygen-enriched liquid vaporized in the heat exchanger to an intermediate level of the second column, optionally following a step of oxygen enrichment of the vaporized liquid, a third column, means for sending at least one condensed portion of the argon-enriched gas in the heat exchanger to the third column, and means for sending an argon-enriched gas flow originating from the second column to the third column, means for withdrawing an argon-enriched flow at the top of the third column, means for sending an argon-depleted liquid from the third column to the second column, means for sending a portion of the oxygen-enriched liquid to an overhead condenser of the third column, and means for sending the vapour produced by vaporizing the oxygen-enriched liquid in the overhead condenser to the second column, characterized in that the means for sending the portion of the oxygen-enriched liquid to the overhead condenser are connected directly to the first column without passing via the exchanger.

According to other optional aspects:

-   -   the apparatus comprises a turbine connected to an intermediate         level of the second column fed with the vaporized,         oxygen-enriched liquid;     -   the apparatus comprises means for returning at least one         condensed portion of the argon-enriched gas to the second         column;     -   the apparatus comprises means for sending the vapour produced to         the second column, by being mixed with the flow expanded in the         turbine;     -   the third column is disposed inside the second column;     -   the apparatus comprises means for vaporizing the at least one         portion of the oxygen-enriched liquid by heat exchange with the         argon-enriched gas inside the second column;     -   the third column contains fewer than 50 or even fewer than 10,         theoretical stages.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will become apparent from the description hereinafter of embodiments, which are given by way of illustration but without any limitation, the description being given in relation with the following attached figures:

FIG. 1 which is composed of FIGS. 1a and 1b , represents comparative methods.

FIG. 2 represents methods according to an embodiment of the invention.

FIG. 3 represents methods according to an embodiment of the invention.

FIG. 4 represents a variant of FIGS. 2 and 3.

FIG. 5 also represents a variant of FIGS. 2 and 3.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1a shows a double air separation column comprising a first column K1, operating at a first pressure, and a second column K2, operating at a second pressure, which is lower than the first pressure. The two columns are connected to one another thermally, for example by a condenser-reboiler C, which vaporizes the bottom oxygen from the second column K2 by heat exchange with the gaseous nitrogen from the first column K1.

A nitrogen-enriched liquid 11 is sent from the top of the first column K1 to the top of the second column K2. The first column is fed with gaseous air by a flow of cooled air 1 which has been purified to remove water and CO2. Air may also feed the second column K2.

An oxygen-enriched liquid is withdrawn at the bottom of the first column K1 and divided into two. One portion 3 is sent to the heat exchanger E, where it is vaporized totally to form a gas 5. The gas 5 is expanded in a turbine T and sent to an intermediate point of the first column K1. The production of cold that is generated at very low temperature by this expansion therefore provides a gain in the energy consumption of the unit, by comparison with the consumption that would occur in the absence of this expansion.

The remainder 10 of the oxygen-enriched liquid withdrawn at the bottom is expanded in a valve and sent as a flow 12 above the intake points of the flows 5 and 9.

The exchanger E, which is contained within a chamber B, is also used to liquefy a flow of intermediate gas 7 from the second column K2. This gas 7 will be withdrawn at a position such that its condensation temperature (bubble point) will be greater than the vaporization temperature of the oxygen-enriched liquid 3 in the exchanger E. Its composition will typically be that of the feed gas of an argon production column. After having undergone condensation in E, this flow is then sent, optionally by means of a pump P, to a point at least above its withdrawal point and below the expanded gas intake of the turbine T.

An oxygen-rich liquid 15 is withdrawn from the bottom of the second column K2, and a nitrogen-enriched top gas 13 is withdrawn from the top of the same column.

As a variant, as illustrated in FIG. 1b , all of the bottom liquid may be sent to the exchanger E, where it undergoes partial vaporization. The partially condensed flow is separated in a phase separator 8 to produce a gas 5 and a liquid 100 which is enriched in oxygen relative to the liquid 3. The resulting gas 5 is expanded in a turbine T, and the remaining liquid 10 is expanded and sent to the column as flow 12. In this case, the liquid enters the column K2 at a level above the gas from the turbine T, since it has been enriched with oxygen. FIG. 1B illustrates only a modified portion of FIG. 1 a.

These diagrams do not include an argon separation column, in contradistinction to FIGS. 2 and 3.

In FIG. 2, which is a variant of FIG. 1, the oxygen-enriched liquid 3 is divided into three portions 3, 17 and 19.

One portion, 17, is sent directly to the second column K2, in liquid form.

The portion 3, as for FIG. 1, undergoes heat exchange with an argon-enriched flow 7, which is a portion of the argon-enriched gas withdrawn from the second column; the remainder of the gas, 7A, is sent directly to feed the argon separation column K3.

The portion 3 is vaporized to form the gaseous flow 5 at 2.1 bar, and is then expanded in the turbine T and sent to the column K2. The flow 7 undergoes condensation in the exchanger E contained in a chamber B, and the resulting liquid 9 feeds the column K3, preferably several stages above the intake of gas 7A.

The chamber B is preferably disposed above the point of intake of the liquid 9 in the column K3.

The portion 19 of the oxygen-enriched liquid feeds the overhead condenser N of the column K3 without having been enriched with oxygen, and undergoes vaporization to form a gas 23. The gas 23 is mixed with gas expanded in the turbine T to form a gas 25, which feeds the second column K2.

Accordingly, the oxygen-enriched liquid feeds the exchanger E and the overhead condenser N in parallel.

The argon yield is of the order of 80%, if argon purified to remove oxygen (flow 21) is recovered as the product. If the flow 21 is not recovered as a pure product, the column K3 can be very small, since it contains only a few tens of theoretical stages (<50), or even fewer than 10 theoretical stages.

In FIG. 3, the oxygen-enriched liquid is divided into only two portions 3 and 3A. The portion 3A feeds the column K2, and the portion 3 is partially vaporized in the heat exchanger E. The remaining liquid, 3B, feeds the overhead condenser N of the column K3, and the gas 23 formed in the condenser feeds the column K2.

The gas 7A formed in the exchanger E feeds the turbine T at an intake pressure of 2.7 bar.

The argon yield is of the order of 75 to 76%, if argon is recovered (flow 21).

In the cases of FIGS. 2 and 3, the argon column has a liquid feed in addition to the usual gaseous feed. Accordingly, the diameter of the column K3 may be reduced by approximately 20%, for the section above the intake of the liquid 9, which reduces its cost.

Given that the argon column is the highest column of the apparatus, it is important to be able to reduce its volume and so to reduce the dimensions of the cold box containing it (not illustrated).

In a variant, the column K3 of FIGS. 2 and 3 may be located inside the column K2, which is disposed concentrically with the shell of the column K2. The column K3 may contain structured packing or dumped packing.

The gas ascending in the column K2 will pass either into the column K3 or into the annular portion surrounding the column K2.

In this case, the overhead condenser N of the column K3 will serve to heat a liquid bath situated at mid-height in the column K2. The gas from the top of the column K3 will pass via a conduit into the overhead condenser N through a barrier forming a tank at mid-height in the column K2, and the liquid condensed in the condenser N will pass in the same way into another conduit, through the barrier, to return to the column K2. A valve may regulate the amount of liquid returned from the condenser N to the column K2.

The column K3 is surrounded by an annular section of the column K2 in which packings are located. The gas separated at the top of the annular section is sent to the section of the column K2, passing through the barrier, into a conduit, or will be sent to the outside of the column, below the barrier, to return into the column above the barrier. The bottom liquid accumulated above the barrier will be sent to the top of the annular section either via a conduit which passes through the barrier or via a conduit which is connected to the outside of the column.

In this case, the exchanger E in its chamber B is still situated outside the column K2 and outside the column K3. In this case, the flow 7 is withdrawn directly from the column K2, without being divided, since the flow equivalent to 7A ascends directly in the column K2 to the column K3.

Similarly, the liquid 3B is injected in the column K2, to be directed to the condenser N.

For a concentric column K3 inside another column K2, since the mixtures of fluids are not identical in composition on either side of the internal column K3, there will be heat exchanges through the wall of the column K2 between the inside of the column K2 and the annular portion. Distillation is promoted by heat exchange at the top of the column K2, whereas it is not promoted by heat exchange at the bottom of the column.

The recommendation is therefore to improve the exchange in the upper part of the column K3 by increasing the surface area for heat exchange, by adding fins on the shell of the upper part of the column K3.

Alternatively, the material used for the upper part of the shell may be a metal with better conductivity than for the lower part (for example, aluminium at the top of the shell of the column K3, and stainless steel at the bottom of the column). Another possibility is to use a shell for K3 that is entirely made of aluminium, and to apply a coating in the lower section in order to reduce heat exchanges.

Proposals have been made in the past to dispose an argon separation column having an overhead condenser in a second column (low-pressure column). One possibility is to position the overhead column such that the top gas from the argon column undergoes partial condensation in the overhead condenser of the argon column and partial condensation in an overhead condenser of the low-pressure column, by heat exchange with the oxygen-rich liquid originating from the bottom of the first column (medium-pressure column). The liquid formed in the overhead condenser of the second column is sent to the top of the second column, and the vaporized liquid is sent to a level above the overhead condenser of the argon column. The overhead condenser may be a film evaporator.

In FIGS. 2 and 3, the turbine T may be replaced by a mixing column K4 operating for example at between 2.2 and 2.7 bar, as illustrated in FIG. 4. This mixing column will be fed at the bottom with the vaporized rich liquid 5 vaporized by the exchanger E. An intake at the top of the column K4 is a flow of impure liquid oxygen having an oxygen content of approximately 90 mol %. The vaporized rich liquid has an oxygen content of 34% in the case of FIGS. 2 and 20% in the case of FIG. 3. A liquid 31 is withdrawn at the bottom of the column K4, that has an oxygen content of 65% (in the case of FIG. 2) or 50% (in the case of FIG. 3). A gaseous flow 43 is withdrawn in the middle of the column K4.

The column K4 produces a flow 35 at the column top that has an oxygen content of 75% (FIG. 2) or 65% (FIG. 3) at between 2.1 and 2.7 bar. This flow is condensed in a condenser C, which may be the bottom condenser of the second column K2 or an evaporator external to any column. It undergoes condensation by heat exchange with the pure liquid oxygen 39, to produce the pure gaseous oxygen 41.

Accordingly, the gas 35 may replace the gaseous nitrogen originating from the first column in the condenser C of FIG. 2 or 3. This allows the argon yield to be increased by approximately 5%, or enables an increase in the production of gaseous nitrogen at the top of the first column.

Conversely, the energy gain would be reduced relative to that of FIGS. 2 and 3; however, the turbomachine T is eliminated.

FIG. 5 also illustrates a variant of FIGS. 2 and 3, in which the oxygen-enriched liquid 3 from the bottom of the first column is enriched with oxygen in an Etienne column K5, the bottom reboiler E of which corresponds to the exchanger 3 in the preceding figures.

Accordingly, the reboiler E is reheated by an argon-enriched gas flow 7 originating from the second argon column. The liquid flow 9 produced is used as a second feed to the argon column K3, in addition to the gaseous feed.

The liquid 3 expanded in a valve descends the stages of the column K5, becoming enriched in oxygen, to produce a oxygen-rich flow 53 (75% oxygen), a bottom flow and an overhead gas containing only 16% oxygen. The flow 53 feeds the column K2 and allows a gain in argon yield of 3%.

As used herein, means for sending a fluid is understood to include one or more conduits and the like that are configured to transfer fluids from one location to another location.

While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.

The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.

“Comprising” in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of “comprising”). “Comprising” as used herein may be replaced by the more limited transitional terms “consisting essentially of” and “consisting of” unless otherwise indicated herein.

“Providing” in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.

Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.

Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range. 

1. A method for separating air by cryogenic distillation, the method comprising the steps of: (a) sending a cooled air that has previous been purified to remove water to a first column operating at a first pressure, where the cooled air is separated into a nitrogen-enriched gas and an oxygen-enriched liquid; (b) withdrawing a liquid enriched in nitrogen relative to the air from the first column and sending said liquid enriched in nitrogen to the top of a second column that is connected thermally to the first column and operates at a second pressure, wherein the second pressure is lower than the first pressure; (c) withdrawing a liquid enriched in oxygen relative to the air from the first column; (d) withdrawing a gas enriched in argon relative to the air from the second column; (e) at least partially vaporizing at least a portion of the oxygen-enriched liquid by heat exchange with the argon-enriched gas to form a vaporized oxygen-enriched fluid, and then sending the vaporized oxygen-enriched fluid to an intermediate level of the second column; (f) returning at least one condensed portion of the argon-enriched gas to a third column, wherein the third column is also fed with an argon-enriched gas flow originating from the second column, wherein an argon-enriched top gas is withdrawn at a top portion of the third column, and an argon-depleted liquid is returned from the third column to the second column; (g) sending a portion of the oxygen-enriched liquid to an overhead condenser of the third column; (h) vaporizing the oxygen-enriched liquid sent to the overhead condenser in the overhead condenser and then sending the resulting oxygen-enriched vapor to the second column, wherein the portion of the oxygen-enriched liquid that is sent to the overhead condenser of the third column has not been reheated against the argon-enriched gas flow.
 2. A method according to claim 1, wherein the vaporized oxygen-enriched fluid is at a pressure at least 1 bar greater than the pressure of the second column, and is expanded in a turbine and then sent to an intermediate level of the second column.
 3. A method according to claim 1, wherein at least one condensed portion of the argon-enriched gas is returned to the second column.
 4. A method according to claim 1, wherein the vapour produced is sent to the second column, by being mixed with the flow expanded in the turbine.
 5. A method according to claim 1, wherein the third column is disposed inside the second column and the at least one portion of the oxygen-enriched liquid is vaporized by heat exchange with the argon-enriched gas inside the second column.
 6. A method according to claim 1, wherein the argon-enriched gas sent to the exchanger in which the heat exchange takes place has a condensation temperature greater than the vaporization temperature of the oxygen-enriched liquid in the exchanger.
 7. An apparatus for separating air by cryogenic distillation, the apparatus comprising: a first column operating at a first pressure; a second column connected thermally to the first column and operating at a second pressure, which is lower than the first pressure; a heat exchanger; means for sending cooled air that has been purified to remove water, to the first column, operating at a first pressure, where the cooled air is separated into a nitrogen-enriched gas and an oxygen-enriched liquid; means for withdrawing a liquid enriched in nitrogen relative to the air from the first column; means for sending the nitrogen-enriched liquid to the top of the second column; means for withdrawing a liquid enriched in oxygen relative to the air from the first column; means for withdrawing a gas enriched in argon relative to the air from the second column; means for sending a portion of the oxygen-enriched liquid to the heat exchanger for at least partial vaporization thereof by heat exchange with the argon-enriched gas; means for sending the oxygen-enriched liquid vaporized in the heat exchanger to an intermediate level of the second column; a third column; means for sending at least one condensed portion of the argon-enriched gas in the heat exchanger to the third column; and means for sending an argon-enriched gas flow originating from the second column to the third column; means for withdrawing an argon-enriched flow at the top of the third column; means for sending an argon-depleted liquid from the third column to the second column; means for sending a portion of the oxygen-enriched liquid to an overhead condenser of the third column; and means for sending the vapour produced by vaporizing the oxygen-enriched liquid in the overhead condenser to the second column; wherein the means for sending the portion of the oxygen-enriched liquid to the overhead condenser are connected directly to the first column without passing via the heat exchanger.
 8. The apparatus according to claim 7, further comprising a turbine connected to an intermediate level of the second column fed with the vaporized oxygen-enriched fluid.
 9. The apparatus according to claim 7, further comprising means for returning at least one condensed portion of the argon-enriched gas to the second column.
 10. The apparatus according to claim 7, further comprising means for sending the vapour produced to the second column, by being mixed with the flow expanded in the turbine.
 11. The apparatus according to claim 7, wherein the third column is disposed inside the second column, and the apparatus further comprises means for vaporizing the at least one portion of the oxygen-enriched liquid by heat exchange with the argon-enriched gas inside the second column.
 12. The apparatus according to claim 7, wherein the third column contains fewer than 50 theoretical stages.
 13. The apparatus according to claim 7, wherein the third column contains fewer than 10 theoretical stages. 